Mathematical models, closely coupled to biologic experimentation, can be developed to (1) describe complex dynamic processes in biomedical research, (2) test hypotheses, (3) improve predictive abilities, (4) extrapolate experimental observations to untested conditions, and (5) minimize the use of experimental animals in an effective manner. The goal of the proposed research is to construct mathematical models capable of predicting changes in the hepatobiliary disposition of xenobiotics during aging. The liver plays a key role in the detoxification (or activation) and elimination of drugs and environmental toxins. Alterations in hepatobiliary xenobiotic disposition during development and aging have been documented for many substrates and species, and are an important consideration in assessing the therapeutic and/or toxic potential of new chemical entities. Previous investigations have tended to be narrow in scope and to rely on intensive animal (primarily mammalian) experimentation. Such studies have provided valuable insights into the mechanism underlying age-dependent changes in hepatic disposition of drugs and toxins, but have demonstrated only a limited ability to extrapolate this mechanistic information to other species or compounds. As described in this proposal, differential equations will be written to model the discrete processes governing hepatobiliary xenobiotic disposition (hepatic uptake, hepatic storage and binding, biotransformation, hepatic egress, biliary excretion and enterohepatic recirculation). These equations will be based upon both biochemical (metabolism, transport) and physiologic (organ volume, blood flow) parameters. Incorporation of physiologic parameters in the model will assist in describing dispositional changes during aging, and will facilitate extrapolation of the model to other mammalian species. Moreover, by providing a well-defined set of parameters (e.g., rate of biotransformation, biliary excretion) that must be determined for any given substrate, application of this model will assist in the design of efficient experiments to elucidate the hepatobiliary disposition of untested compounds. Hepatobiliary xenobiotic disposition (including enterohepatic recycling) is too complex to be simulated comprehensively by mathematical models, or evaluated solely by in vitro techniques, without the aid of mammalian systems. The Fischer-344 rat (at selected ages) will serve as the basis for development of the mathematical model. Two prototype substrates, valproic acid and morphine, representing two distinct classes of chemical compounds, will be used in model development. A multiexperimental approach, employing various in vitro (cellular and subcellular) and in situ techniques, will be utilized to examine each process involved in hepatobiliary xenobiotic disposition and to obtain parameter estimates for the mathematical model. Model testing and validation in an appropriate mammalian system is an essential part of developing mathematical models of biologic systems that pertain to human physiology and/or disease. The ability of the model to predict correctly the disposition of each substrate will be tested in limited in vivo experiments. Data available in the literature for both prototypes will be used to assess the capability of the model to extrapolate to other species. In summary, the mathematical models developed in this project will maximize the use of data collected from in vitro systems (e.g., human hepatocytes). More importantly, these models will provide a method to translate mechanistic knowledge about age-dependent alterations in discrete processes governing hepatobiliary disposition into rational extrapolations of hepatic xenobiotic disposition across developmental stages and species (including human), and will reduce the number of vertebrate animals required for future investigations utilizing other substrates and/or animal species.